It is well known in the art that the types and amounts of substances present in exhaust gases from an internal combustion engine are greatly affected by the ratio of air to fuel in the mixture supplied to the engine. Rich mixtures, with excess fuel, tend to produce higher amounts of hydrocarbons and carbon monoxide, whereas lean mixtures, with excess air, tend to produce greater amounts of oxides of nitrogen. It is also well known in the art that exhaust gases can be treated so as to reduce the amounts of these undersirable components. An example of this treatment is catalytic treatment in which carbon monoxide and hydrocarbons are oxidized and nitrogen oxides are reduced.
Such treatment can be achieved by providing a catalytic converter and/or a reactor in the exhaust system of the engine. It has been suggested that oxidation and reduction for minimization of undesirable constituents is preferably performed when the air-fuel ratio of the exhaust gases upstream of the converter or reactor is maintained within a narrow range at, for example, stoichiometry.
In order to maintain the air-fuel ratio of the exhaust gases upstream of the converter or reactor, various air-fuel control systems have been developped. A typical approach utilizes a detector for detecting the air-fuel ratio of the exhaust gases upstream of the converter etc., a control signal generator for generating a control signal varying in amplitude in accordance with variations in the air-fuel ratio of the exhaust gases, and an air-fuel ratio regulator for regulating the air-fuel ratio of the air-fuel mixture to be supplied to the engine in response to the control signal.
Since variations in the air-fuel ratio of the exhaust gases appear after an appreciably long delay in the exhaust gases, the control signal undergoes undesirable hunting about a central level corresponding to a desired air-fuel ratio of the exhaust gases. In order to suppress the amplitude in this hunting of the control signal, various techniques have been utilized with the control system. However, difficulty is still encountered in conventional air-fuel ratio control systems.